1. Field of the Invention
The present invention generally relates to a miniaturized microstrip antenna, in particular, to a coupled-loop chip antenna.
2. Description of the Prior Art
With rapid development of wireless communications recently, increasingly high requirements on the quality of the wireless communication continuously come up. Antenna is a dispensable communication element for all the wireless applications. In different wireless communication applications, although the requirements on the characteristics of the antenna are different, it can be summarized that, in order to obtain a stable communication quality, the antenna is required to be a communication element having various electrical characteristics such as multiple frequency bands, appropriate bandwidths (depending upon the type of the wireless communication application), a high integrity, a low cost, and a small size. Among the selections of various antenna structures, the loop antenna is one structure adopted by the antennas of various wireless communication products.
The loop antenna is an alternation of a unipole antenna. The conventional unipole antenna is a linear structure, which has a length of at least one fourth of the frequency to be radiated, and is vertical to the vast ground surface. The structure can be easily fabricated but the antenna has the disadvantages including an overly narrow bandwidth, a low integrity, and a large size. The loop antenna makes an improvement on the antenna body of the conventional unipole antenna. For example, Taiwan Patent Publication No. I270236 entitled “Loop Antenna Having Capacitive Structure” has disclosed a loop antenna having a capacitive structure. The loop antenna body is a linear structure with a plurality of three-dimensional bends, and is close and parallel to the vast ground surface for generating coupled capacitance, so as to reduce the size of the antenna and increase the bandwidth. However, the body of the loop antenna having a capacitive structure is a metal line segment with a plurality of three-dimensional bends, and is close to the ground, thus causing a high cost during the fabricating process. Moreover, due to the special three-dimensional bend structure, the antenna in the wireless application product needs to be processed specially to integrate with other structures, and thus has no integrity. In brief, Taiwan Patent Publication No. I270236, entitled “Loop Antenna Having Capacitive Structure,” indeed solves the problems in the conventional unipole antenna and the loop antenna, but still fails to provide an antenna having various electrical characteristics including multiple frequency bands, appropriate bandwidth, a high integrity, a low cost, and a small size.
In addition, the chip antenna with a high integrity is a common implementation of antenna, but the characteristics of the antenna applying the chip process technique are always limited to an overly small chip size, for example, the size of the fabricated chip usually is in the range of millimeter (mm). Therefore, the antenna structure in the range of millimeters may generate an unnecessary electric field and magnetic field coupling with the metal wire on other layers or other metal structures, which damages the original antenna performance and causes a narrow operating bandwidth. The bandwidth of the conventional chip antenna is about 30 MHz, which is too narrow for the chip antenna to be applied on various wireless communication devices effectively.